Purification catalyst for exhaust gas, production method therefor, and purification catalyst equipment for exhaust gas

ABSTRACT

A purification catalyst for exhaust gas which exhibits satisfactory performance even at a low temperature operation of starting or idling of engine (not more than 400° C.), and a production method therefor are provided. 
     The catalyst comprises an aluminum oxide supporting Pd, and the aluminum oxide is LnAlO 3  (Ln: rare-earth metal).

TECHNICAL FIELD

The present invention relates to a purification catalyst for exhaustgas, to a production method therefor, and to purification catalystequipment for exhaust gas, and specifically relates to a productiontechnique of a purification catalyst for exhaust gas in which nitrogenoxides (NOx), carbon hydride (HC), and carbon monoxide (CO) contained ina exhaust gas emitted from an internal combustion engine (for example,in a vehicle) can be simultaneously and effectively reduced, therebyreducing the undesirable components of the exhaust gas.

BACKGROUND ART

For purifying exhaust gas containing, for example, CO, HC, and NO,precious metal elements (Pt, Rh, Pd, and Ir) exhibit a high performance.Therefore, it is preferable to employ the above-mentioned precious metalelements to the purification catalyst for exhaust gas. These preciousmetals are generally supported by Al₂O₃ which is a support having a highsurface-to-weight ratio. On the other hand, composite oxides (forexample, a perovskite-like oxide) made by combining various elementshave extremely varied properties. Therefore, it is preferable for apurification catalyst for exhaust gas to employ the above-mentionedcomposite oxides. Moreover, when the precious metal is supported by thecomposite oxides, the properties of the precious metal are significantlychanged. From this viewpoint, a preferable performance for purifyingexhaust gas can be obtained in the purification catalyst for exhaust gasin which a precious metal is supported by a composite oxide.

Various catalysts mentioned above are now developed, and for example, atechnique in which a coalescence rate of the precious metal can bereduced by setting a perovskite-like composite oxide to be a support,judging from deterioration of the precious metal with reduction ofactive sites by coagulation of the precious metal, is proposed (see theclaims of Japanese Unexamined Application Publication No. 5-86259).Moreover, another technique in which reduction of PdO can be reduced byusing a perovskite-like composite oxide in which the A site isdefective, judging from reducing PdO which is an activated species in aNO reduction reaction, whereby the PdO changes to Pd which is low-activePd, when the precious metal is Pd, is proposed (see the claims ofreacting of Japanese Unexamined Application Publication No.2003-175337).

Conventional purification catalysts for exhaust gas exhibit sufficientperformance for reducing CO, HC, and NOx contained in exhaust gas, in arunning of vehicle, particularly during a running at high temperatures(not less than 400° C.). However, the conventional catalysts cannotexhibit sufficient performance for reducing CO, HC, and NOx, in avehicle at the starting or idling thereof at low temperatures (not morethan 400° C.).

As mentioned above, the reason that sufficient performance for purifyingthe exhaust gas cannot be obtained in the running at low temperature isas follows. That is, in the conventional purification catalyst forexhaust gas, precious metal, for example, Pt, Rh, or Pd, is supported onAl₂O₃ having a high surface-to-weight ratio. Due to the highsurface-to-weight ratio of the Al₂O₃, the precious metal isadvantageously supported in a highly dispersed condition. However, Al₂O₃is a stable compound, and does not mutually affect a supported preciousmetal, whereby activity of the precious metal is not improved.Accordingly, sufficient performance during the running at lowtemperature cannot be obtained.

Moreover, in the running of a vehicle, it is preferable for Pd to existin a condition of PdO which is highly reactive. However, even if Pdsupported on the Al₂O₃ initially exists in a condition of PdO, the Pd isreduced to be a metal condition at high temperatures, whereby theactivity is significantly reduced.

DISCLOSURE OF INVENTION

The invention was made in light of the above demands, and it is hence anobject thereof to provide a purification catalyst for exhaust gas, inwhich activity of the precious metal is improved, and the reduction ofactivity at high temperatures is prevented, whereby sufficientperformance even in a vehicle starting up or idling at low temperatures(not more than 400° C.) can be obtained, and a production methodtherefor, and a purification catalyst equipment for exhaust gas.

The present inventors have intensively researched purification catalystsfor exhaust gas, in which sufficient performance, even in a vehiclestarting up or idling at low temperatures (not more than 400° C.), canbe exhibited. Consequently, it has been learned that a purificationcatalyst for exhaust gas made by supporting Pd on LnAlO₃ (Ln: rare-earthmetal) has an effect of suppressing a reduction of PdO to Pd at a hightemperature, whereby in the above-mentioned catalyst the high activitycan be maintained during the running at low temperatures after runningat high temperatures.

The present invention (the first aspect of the invention) was made inlight of the above knowledge. That is, a purification catalyst forexhaust gas of the present invention is a catalyst in which Pd issupported on an aluminum oxide, and the oxide is LnAlO₃ (Ln: rare-earthmetal).

Moreover, the present inventors have also learned that a LaAlO₃ amongLnAlO₃ compounds, is trigonal or rhombohedral, and a B site in theperovskite-like composite oxide is Al in the LaAlO₃, whereby dipolemoment of the LaAlO₃ is large, and an electric fluctuation of PdObounded on the LaAlO₃ is larger than that of PdO which existsindependently. Therefore, the oxidation state of Pd in a surface of thePdO supported is a state of Pd²⁺ over a large area. This state is apreferable state for purifying exhaust gas, whereby high activity at lowtemperatures can be obtained. Additionally, the present inventors haveconfirmed that this catalyst can exhibit high activity at lowtemperatures even after exposing the catalyst to operating conditions ofabout 1000° C.

The present invention (the second aspect of the invention) was made inlight of the above knowledge. That is, in the above-mentionedpurification catalyst for exhaust gas (the first invention), it ispreferable that the aluminum oxide be trigonal or rhombohedral.

Furthermore, the present inventors have also learned that when LnAlO₃ isproduced, an aqueous nitrate solution of a component containing aqueouscarboxylic acid may be evaporated completely to obtain a carboxylic acidcomplex polymer, whereby LnAlO₃ is generated as a single phase, and asurface of the LnAlO₃ supporting Pd changes to a configuration in whichinteraction with PdO is easy.

The present invention (the third and fourth aspects of the inventions)was made in light of the above knowledge. That is, in theabove-mentioned purification catalysts for exhaust gas (the first andsecond aspects of the invention), it is preferable that at least onekind of compound selected from a group of compounds (carboxylic acidhaving a hydroxyl group or a mercapto group and having a carbon numberof 2 to 20, dicarboxylic acid having a carbon number of 2 or 3, andmonocarboxylic acid having a carbon number of 1 to 20) be added toaqueous nitrate solution including a component, whereby a purificationcatalyst for exhaust gas is obtained (the third aspect of theinvention). Moreover, in the purification catalysts for exhaust gas (thethird aspect of the invention), it is preferable that the aqueousnitrate solution be evaporated completely to obtain a carboxylic acidcomplex polymer, and that the carboxylic acid complex polymer be heated,whereby a purification catalyst for exhaust gas is obtained (the fourthaspect of the invention).

As the carboxylic acid having a hydroxyl group or a mercapto group andhaving a carbon number of 2 to 20, oxycarboxylic acid and a compound inwhich an oxygen atom in the hydroxyl of the oxycarboxylic acid isreplaced with a sulfur atom are cited. The carbon number of thesecarboxylic acids is 2 to 20 in light of solubility in water, ispreferably 2 to 12, is more preferably 2 to 8, and is most preferably 2to 6. Moreover, the carbon number of the monocarboxylic acid is 1 to 20in light of solubility in water, is preferably 1 to 12, is morepreferably 1 to 8, and is most preferably 1 to 6.

Furthermore, as concrete examples of the carboxylic acids having ahydroxyl group or a mercapto group and having a carbon number of 2 to20, for example, glycolic acid, mercaptosuccinic acid, thioglycolicacid, lactic acid, β-hydroxy propionic acid, malic acid, tartaric acid,citric acid, isocitric acid, allo-citric acid, gluconic acid, glyoxylicacid, glyceric acid, mandelic acid, tropic acid, benzylic acid, andsalicylic acid are cited. As concrete examples of the monocarboxylicacids, for example, formic acid, acetic acid, propionic acid, butyricacid, isobutyric acid, valeric acid, isovaleric acid, hexanoic acid,heptanoic acid, 2-methyl hexanoic acid, octanoic acid, 2-ethyl hexanoicacid, nonanoic acid, decanoic acid, and lauric acid are cited. In theabove-mentioned acids, it is preferable to use acetic acid, oxalic acid,malonic acid, glycolic acid, lactic acid, malic acid, tartaric acid,glyoxylic acid, citric acid, gluconic acid, and more preferable to useoxalic acid, malonic acid, glycolic acid, lactic acid, malic acid,tartaric acid, glyoxylic acid, citric acid, gluconic acid.

Additionally, the present inventors have particularly researched indetail about a purification catalyst for exhaust gas in which rare-earthmetal is applied to an A site of perovskite-like composite oxides.LnAlO₃ (Ln: rare-earth metal) is trigonal or rhombohedral. Therefore,the electron state is extremely unstable. Moreover, in these oxides, Alis applied to a B site of perovskite-like composite oxides, whereby adipole moment exists due to the strong covalent bond between Al and O.Therefore, the dipole moment of LnAlO₃ is larger than that of theconventional purification catalyst for exhaust gas, for example LaFeO₃.Owing to the properties of LnAlO₃, an electric fluctuation of PdObounded on the LaAlO₃ is larger than that of PdO which existsindependently, the oxidation state of Pd in a surface of the PdO whichis supported is a Pd²⁺ state over a large area. Generally, Pd in thesurface of the PdO exists in two states of Pd²⁺ and Pd⁰ (metal state).In these states, a state of Pd²⁺ has higher activity as a purificationcatalyst for exhaust gas than a state of Pd⁰. That is, a purificationcatalyst for exhaust gas of the present invention, in which Pd issupported on the LnFeO₃, has high activity, because most of the Pd onthe surface of the PdO exists in a state of Pd²⁺. Moreover, thesecatalysts can equally maintain the high activity state even after thecatalysts are exposed during use to conditions of 1000° C.

The present invention (the fifth aspect of the invention) was made inlight of the above knowledge. That is, in the above-mentionedpurification catalysts for exhaust gas (the second to fourth aspect ofthe invention), it is preferable that Pd be supported on the LnAlO₃ (Ln:rare-earth metal), and that Pd exist in a state of Pd²⁺ in the surfacerange that the Pd is supported (the fifth aspect of the invention).

Next, a production method for a purification catalyst for exhaust gas ofthe present invention (the sixth aspect of the invention) is a methodfor preferably producing the above-mentioned catalysts (the first tofifth aspects of the invention). That is, the sixth aspect of theinvention is a method in which when the purification catalyst forexhaust gas in which Pd is supported on an aluminum oxide, at least onekind of compound selected from a group of compounds (carboxylic acidhaving a hydroxyl group or a mercapto group and having a carbon numberof 2 to 20, a dicarboxylic acid having a carbon number of 2 or 3, and amonocarboxylic acid having a carbon number of 1 to 20) is added toaqueous nitrate solution including a component, whereby a purificationcatalyst for exhaust gas is obtained.

In the above-mentioned production method for a purification catalyst forexhaust gas (the sixth aspect of the invention), it is preferable thatthe aqueous nitrate solution be evaporated completely to obtain acarboxylic acid complex polymer, and that the carboxylic acid complexpolymer be heated (the seventh aspect of the invention), and it is morepreferable that the heating temperature be not more than 1000° C. (theeighth aspect of the invention).

Furthermore, purification catalyst equipment for exhaust gas (the ninthaspect of the invention), produced by using the above-mentionedpurification catalysts for exhaust gas (the first to fifth aspects ofthe invention), is desirable for internal combustion, for example, in avehicle, particularly because nitrogen oxides (NOx), carbon hydride(HC), and carbon monoxide (CO) contained in an exhaust gas can besimultaneously and effectively reduced by reducing by the equipment.

The purification catalyst for exhaust gas of the present invention inwhich Pd is supported on LnAlO₃ has a function in which the reduction ofPdO to Pd metal can be reduced. The shape of Ln (rare-earth metal)variously changes in oxide states. For example, when a catalyst made bysupporting Pd on La₂O₃ is exposed to high temperature conditions, La₂O₃migrates onto the Pd grain from the contact area between Pd and La₂O₃,whereby a shape of filling up La₂O₃ with Pd is formed, resulting inadditional migration of minute amounts of La₂O₃ onto the Pd surface(Zhang et al., J. Phys. Chem., Vol. 100, No. 2, pp. 744-755, 1996). Evenin the present system (LnAlO₃), Ln and Pd form a complex compound,whereby reduction of PdO to Pd metal can be reduced. Owing to thiseffect, a purification catalyst for exhaust gas of the present inventioncan maintain the high activity state while running at low temperatures(not more than 400° C.).

Moreover, in the LnAlO₃, for example LaAlO₃ (including Pd/PrAlO₃ orPd/NdAlO₃) is characterized in that the crystal system is trigonal orrhombohedral. The trigonal or rhombohedral is, as shown in FIG. 1, acrystal system in which an ideal cubic system of a unit lattice ischanged in the c-axis direction, and the angle between the a-axis andthe b-axis is 120°. That is, the trigonal or rhombohedral is a crystalsystem in which an ideal cubic system of a perovskite structure issignificantly strained. In the crystal system, the electrons state amongconstituent atoms is extremely unstable. FIG. 2 is a graph showing a XRDspectrum as data to confirm the differences of the crystal systems ofthe LaAlO₃ supporting Pd, etc. That is, when Pd/LaAlO₃, Pd/PrAlO₃,Pd/NdAlO₃, and other perovskite-like composite oxides supporting Pdwhich is a conventional purification catalyst for exhaust gas(Pd/GdAlO₃, Pd/LaCoO₃, Pd/LaFeO₃, and Pd/LaMnO₃) are compared, adifference in strength of the main peak and deviance of position inother peaks are seen in the FIG. 2. Accordingly, judging from the factthat LaAlO₃, PrAlO₃, or AlO₃ is trigonal or rhombohedral, otherperovskite-like composite oxides (GdAlO₃, LaCoO₃, LaFeO₃, or LaMnO₃) arenot trigonal or rhombohedral, but are rhombic. Additionally, in theconventional purification catalyst for exhaust gas, LaNiO₃ does not havea difference in strength at a main peak and deviance of position inother peaks against LaAlO₃, whereby LaNiO₃ is trigonal or rhombohedral.

On the other hand, in the LaAlO₃, PrAlO₃, and NdAlO₃, a B site in theperovskite-like composite oxide is Al, whereby the bond between Al and Ohas a high degree of probability of being a covalent bond. Therefore,some of the dipole moment is generated in a crystal of perovskite-likecomposite oxides which has generally a high degree of probability ofbeing an ionic bond. As described above, the perovskite-like compositeoxides, that is LaAlO₃, PrAlO₃, and NdAlO₃, are trigonal orrhombohedral, and a B site in the perovskite-like composite oxides is Alin the oxides, whereby dipole moment of the oxides is larger than thatof the well-known purification catalyst for exhaust gas, for exampleLaFeO₃.

Due to the dipole moment, an electric fluctuation of PdO bounded on theLaAlO₃, PrAlO₃, and NdAlO₃ is larger than that in which PdO existsindependently. Therefore, the oxidation state of Pd in a surface of thePdO supported is a state of Pd²⁺ over a large area. There are twooxidation states of Pd in a surface of the PdO, which are a state ofPd²⁺ and a state of Pd⁰ (metal state). The state of Pd²⁺ was higheractivity than the state of Pd⁰. That is, in the purification catalystsfor exhaust gas of the present invention in which Pd is supported on theLaAlO₃, PrAlO₃, and NdAlO₃, the oxidation state of Pd in a surface ofthe PdO is the state of Pd²⁺, whereby the catalysts of the presentinvention have high activity. Moreover, the catalysts of the presentinvention can exhibit high activity during the running at lowtemperatures (not more than 400° C.) even after exposing the catalyst toan operating condition of about 1000° C.

Furthermore, when the LaAlO₃, PrAlO₃, or NdAlO₃ is produced, an aqueousnitrate solution of a component containing carboxylic acid is evaporatedcompletely to obtain a carboxylic acid complex polymer, and the polymeris heated at a relatively low temperature of 800° C., whereby LaAlO₃,PrAlO₃, or NdAlO₃ are generated as a single phase. On the other hand,when the LaAlO₃, PrAlO₃, or NdAlO₃ is produced in other ways, forexample, solid-phase reaction, LaAlO₃, PrAlO₃, or NdAlO₃ is notgenerated as a single phase even if the heating at a relatively hightemperature of 1700° C. is performed (see Rare Earth Science,Kagaku-Dojin Publishing Company, Inc, Ginya Adachi, p. 564). That is,LaAlO₃, PrAlO₃, or NdAlO₃ of the single phase can be synthesized at theabove-mentioned low temperature by using carboxylic acid. Therefore,sufficient surface-to-weight ratio can be obtained, and the catalyst canbe used in a state in which the surface of the crystal lattice isactive. In the purification catalyst for exhaust gas made by supportingPd on the LnAlO₃ by using the method of the present invention,sufficient surface-to-weight ratio and strong interaction between LnAlO₃and Pd can be obtained, whereby high activity at low temperatures can berealized.

As mentioned above, the LnAlO₃(Ln: rare-earth metal) is trigonal orrhombohedral, whereby the electrons state among constituent atoms inLnAlO₃ is extremely unstable, and the bond among the Al and the O is astrong covalent bond, whereby some of dipole moment is generated.Therefore, most of Pd supported on these oxides exists in a state ofPd²⁺. In order to confirm this, in the Pd/LaAlO₃ which is arepresentative of the present invention and in the Pd/LaFeO₃ andPd/Al₂O₃ which is a representative of the conventional technique, statesof Pd in a PdO surface were examined by XPS. Generally, a peak positionof the metal component (Pd⁰) of Pd is 335.5±0.3 eV. On the other hand, apeak position of the ion component (Pd²⁺) of Pd is 336.6±0.4 eV. Judgingfrom this fact and the results shown in FIG. 3, in the Pd/LaAlO₃, thereis a peak at a position which is equivalent to the Pd²⁺, in thePd/LaFeO₃ and Pd/Al₂O₃, there is a peak at a position which isequivalent to the Pd⁰. Accordingly, states of Pd in a surface of theLaAlO₃ are mostly Pd²⁺. Additionally, the states of Pd in a surface ofthe LaAlO₃ are confirmed as mentioned above, whereby states of Pd in asurface of the other LnAlO₃ (Ln: rare-earth metal), for example PrAlO₃and NdAlO₃ are similarly estimated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective illustration showing a crystal system of LaAlO₃constituting a purification catalyst for exhaust gas of the presentinvention.

FIG. 2 is a graph showing a XRD spectrum as data to confirm thedifferences of the crystal systems of the LaAlO₃ supporting Pd, etc.

FIG. 3 is a graph showing 3d orbital vicinity of Pd examined regardingthe Pd states in a PdO surface by XPS, in the Pd/LaAlO₃ which is anexample of the present invention and in the Pd/LaFeO₃ and Pd/Al₂O₃ whichis an example of the conventional technique.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be concretely explained byexamples.

Practical Examples 1 to 3 Production of Composite Oxides as Support

Predetermined amounts of lanthanum nitrate hexahydrate and aluminumnitrate nonahydrate were dissolved in ion-exchanged water, whereby amixed solution was obtained. Next, a predetermined amount of malic acidwas dissolved in ion-exchanged water, whereby an aqueous malic acidsolution was obtained. These two solutions were mixed, the obtainedmixed solution was set on a hot plate with a stirrer, and the mixedsolution was heated to 250° C. and agitated by a stirring bar, wherebyevaporation of water into vapor was performed, complete evaporation wasperformed, and the dried sample was crushed into powder by mortar andpestle. The crushed sample was moved to an aluminum crucible, the samplewas heated to 350° C. at a rate of 2.5° C./min in a muffle kiln, and aheat treatment was performed at 350° C. for 3 hours. Owing to the heattreatment, a provisional heated substance in which malate andnitrate-nitrogen (nitrate salt and nitrate ion) were removed wasobtained. After crushing the provisional heated substance into powderand mixing for 15 minutes by a mortar and pestle, the obtained mixturewas set in the aluminum crucible again, the sample was heated to 800° C.at a rate of 5° C./min in the muffle kiln, and a heat treatment wasperformed at 800° C. for 10 hours. Owing to the heat treatment, aperovskite-like composite oxide of which the composition was LaAlO₃ wasobtained. Moreover, perovskite-like composite oxides of whichcompositions are PrAlO₃ and NdAlO₃ were similarly obtained.

[Supporting of Precious Metal]

Next, a predetermined amount of palladium nitrate dehydrate wasdissolved in ion-exchanged water, whereby an aqueous palladium nitratesolution was obtained. The palladium nitrate and a predetermined amountof LaAlO₃, PrAlO₃, or NdAlO₃ which was in powder form were set in aflask which was like an eggplant, and the sample was completely dried ina hot water bath at 60° C. while decreasing pressure in the flask by arotary evaporator. After that, the sample was heated to 250° C. at arate of 2.5° C./min in a muffle kiln, was heated to 750° C. at a rate of5° C./min, and was held at 750° C. for 3 hours. Due to these treatments,catalyst powders of the Practical Examples 1 to 3, of which thecompositions were Pd/LaAlO₃, Pd/PrAlO₃, and Pd/NdAlO₃, in which PdO wasimpregnated and supported on the perovskite-like composite oxides, wereobtained.

Surface-to-weight ratios for these catalyst powders are shown in Table1.

TABLE 1 Sample No. Composition Surface-to-Weight Ratio (m²/g) PracticalExample 1 Pd/LaAlO₃ 9 Practical Example 2 Pd/PrAlO₃ 8 Practical Example3 Pd/NdAlO₃ 8 Comparative Example 1 Pd/Al₂O₃ 80 Comparative Example 2Pd/GdAlO₃ 9 Comparative Example 3 Pd/LaNiO₃ 5 Comparative Example 4Pd/LaMnO₃ 15 Comparative Example 5 Pd/LaCoO₃ 4 Comparative Example 6Pd/LaFeO₃ 5 Comparative Example 7 Pd/LaAlO₃ 1

[Estimation of Activity]

Next, initial activities and activities after endurance running wereestimated for the obtained catalyst powders. The estimation wasperformed by flowing model exhaust gas of a vehicle into catalysts underconditions in which A/F (air-fuel ratio) was substantially 14.6 and SV(stroke volume) was 5000 h⁻¹. Endurance running was performed for 20hours at an endurance running temperature of 900° C. by using modelexhaust gas in which A/F (air-fuel ratio) was substantially 14.6. Theseresults are shown in FIG. 2 and FIG. 3. That is, the FIG. 2 shows atemperature at which CO, HC, and NO are reduced by 50% in a temperatureincrease test of catalysts before the endurance running. Moreover, theFIG. 3 shows a temperature at which CO, HC, and NO are reduced by 50% ina temperature increase test of catalysts after the endurance running.

TABLE 2 Temperature for 50% Reduction (° C.) Sample No. Composition COHC NO Practical Example 1 Pd/LaAlO₃ 254 260 197 Practical Example 2Pd/PrAlO₃ 251 256 201 Practical Example 3 Pd/NdAlO₃ 258 264 201Comparative Example 1 Pd/Al₂O₃ 288 295 302 Comparative Example 2Pd/GdAlO₃ 273 280 213 Comparative Example 3 Pd/LaNiO₃ 299 315 217Comparative Example 4 Pd/LaMnO₃ 281 299 204 Comparative Example 5Pd/LaCoO₃ 305 320 233 Comparative Example 6 Pd/LaFeO₃ 300 305 241Comparative Example 7 Pd/LaAlO₃ 292 301 235

TABLE 3 Endurance Running Temperature for 50% Reduction (° C.) SampleNo. Composition Temperature(° C.) CO HC NO Practical Example 1 Pd/LaAlO₃900 317 324 260 Practical Example 2 Pd/PrAlO₃ 900 312 323 263 PracticalExample 3 Pd/NdAlO₃ 900 318 332 279 Comparative Example 1 Pd/Al₂O₃ 900326 335 >400 Comparative Example 2 Pd/GdAlO₃ 800 339 355 >400Comparative Example 3 Pd/LaNiO₃ 800 328 352 354 Comparative Example 4Pd/LaMnO₃ 800 309 320 321 Comparative Example 5 Pd/LaCoO₃ 900 329 354278 Comparative Example 6 Pd/LaFeO₃ 900 365 363 >400 Comparative Example7 Pd/LaAlO₃ 900 332 361 295

Comparative Example 1

Pd/Al₂O₃ was produced in a manner similar to that of the PracticalExample 1, and various estimations for activity were performed. Theendurance running temperature was set at 900° C. The result was alsoshown in Table 1 to 3.

Comparative Example 2

Pd/GdAlO₃ was produced in a similar manner with the Practical Example 1.The crystal system of the GdAlO₃ is rhombic. Various estimates ofactivity were performed for this catalyst. The endurance runningtemperature was set at 900° C. The results are also shown in Tables 1 to3.

Comparative Example 3

Pd/LaNiO₃ was produced in a manner similar to that of the PracticalExample 1. The LaNiO₃ is trigonal or rhombohedral. Various estimationsfor activity were performed for this catalyst. The endurance runningtemperature was set at 800° C. The results are also shown in Tables 1 to3.

Comparative Example 4

Pd/LaMnO₃ was produced in a manner similar to that of the PracticalExample 1. The crystal system of the LaMnO₃ is rhombic. Variousestimations for activity were performed for this catalyst. The endurancerunning temperature was set at 800° C. The results are also shown inTables 1 to 3.

Comparative Example 5

Pd/LaCoO₃ was produced in a manner similar to that of the PracticalExample 1. The crystal system of the LaCoO₃ is rhombic. Variousestimations for activity were performed for this catalyst. The endurancerunning temperature was set at 800° C. The results are also shown inTables 1 to 3.

Comparative Example 6

Pd/LaFeO₃ was produced in a manner similar to that of the PracticalExample 1. The crystal system of the LaFeO₃ is rhombic. Variousestimations for activity were performed for this catalyst. The endurancerunning temperature was set at 900° C. The results are also shown inTables 1 to 3.

Comparative Example 7

A given amount of lanthanum oxide and aluminum oxide were mixed bymortar and pestle, the mixed sample was moved to an aluminum crucible,the sample was heated for 10 hours at 1100° C. in a muffle kiln, andLaAlO₃ was obtained by solid-phase reaction. A precious metal wassupported in a similar manner of the Practical Example 1 by using theLaAlO₃, whereby Pd/LaAlO₃ was obtained. Various estimations for activitywere performed for this catalyst. The endurance running temperature wasset at 900° C. The results are also shown in Tables 1 to 3.

According to the Tables 2 and 3, the purification catalysts for exhaustgas of the Practical Example 1 to 3 exhibit excellent temperatures atwhich CO, HC, and NO are reduced by 50% at any time before and after theendurance running. The reason for this is that the purificationcatalysts for exhaust gas of the Practical Examples 1 to 3 are made bysupporting Pd on the LaAlO₃, PrAlO₃, or NdAlO₃, and these catalysts havea property of suppressing a reduction of PdO to Pd at high temperatures,whereby the high activity can be maintained in the running at lowtemperatures after a running at high temperatures in the catalysts.Moreover, the purification catalysts for exhaust gas of the PracticalExamples 1 to 3 are trigonal or rhombohedral, and a B site in theperovskite-like composite oxide is Al in the catalysts of the PracticalExamples 1 to 3, whereby dipole moment of the catalysts is large.Therefore, an electric fluctuation of PdO bounded on the LaAlO₃, PrAlO₃,or NdAlO₃ is larger than that of PdO which exists independently.Furthermore, in the purification catalysts for exhaust gas of thePractical Examples 1 to 3, LaAlO₃, PrAlO₃, or NdAlO₃ is produced,aqueous nitrate solution of element containing carboxylic acid isevaporated completely to obtain carboxylic acid complex polymer, wherebyLaAlO₃, PrAlO₃, or NdAlO₃ is generated as a single phase, and a surfaceof the LaAlO₃, PrAlO₃, or NdAlO₃ supporting Pd take a form in whichinteraction with PdO is easy. Additionally, when the mixed solution isproduced, malic acid can be used as mentioned above, and when citricacid and oxalic acid are similarly used, the same effect can beobtained.

On the other hand, the purification catalysts for exhaust gas of theComparative Examples 1 to 7 cannot exhibit an excellent temperature atwhich CO, HC, and NO are reduced by 50% at any time before and after theendurance running. The reason is as follows. That is, in the catalyst ofthe Comparative Example 1, Al₂O₃ is a stable compound, and Al₂O₃ doesnot mutually affect precious metal supported, whereby activity of Pddoes not improve. In the catalyst of the Comparative Example 2, thecrystal system is rhombic, whereby electrons state among constituentatoms is not more unstable compared with the case of trigonal orrhombohedral. In the catalyst of the Comparative Example 3, even thoughthe crystal system is trigonal or rhombohedral, Al does not exist in a Bsite in the perovskite-like composite oxide, whereby it is difficult togenerate some of dipole moment in a crystal of perovskite-like compositeoxides which generally has a high degree of probability of being anionic bond. In the catalyst of the Comparative Examples 4 to 6, thecrystal systems are rhombic, whereby electrons states among constituentatoms are not more unstable compared with the case of trigonal orrhombohedral. In the catalyst of the Comparative Example 7, carboxylicacid is not used when the catalyst is produce, whereby LaAlO₃ cannot begenerated as a single phase. Therefore, sufficient surface-to-weightratio cannot be obtained, and the surface of the crystal lattice cannotbe used in an active state.

The purification catalyst for exhaust gas of the present invention canbe applied to an internal combustion engine of vehicles in whichnitrogen oxide (NOx), hydrocarbon (HC) and carbon monoxide (CO) inexhaust gas are required to be simultaneously and effectively purifiedand reduced recently.

1. A purification catalyst for exhaust gas comprising an Al oxidesupporting Pd and PdO, where the Al oxide is (Ln: rare-earth metal)generated as a single phase and trigonal or rhombohedral.
 2. Apurification catalyst for exhaust gas according to claim 1 wherein thecatalyst is produced by adding at least one kind of compound selectedfrom the group of compounds of carboxylic acid having a hydroxyl groupor a mercapto group and having a carbon number of 2 to 20, dicarboxylicacid having a carbon number of 2 or 3, and monocarboxylic acid having acarbon number of 1 to 20 to aqueous nitrate solution including Ln andAl.
 3. The purification catalyst for exhaust gas according to claim 2,wherein the catalyst is produced by evaporating the aqueous nitratesolution completely, to produce a carboxylic acid complex polymer andheating the carboxylic acid complex polymer.
 4. The purificationcatalyst for exhaust gas according to claim 3, wherein Pd is supportedon LnAlO₃ in which Ln is a rare-earth metal, and an oxidation state ofPd in a surface supporting Pd is a state of Pd²⁺.
 5. A Purificationcatalyst equipment for exhaust gas, comprising the purification catalystfor exhaust gas according to claim 1 or
 2. 6. The purification catalystfor exhaust gas according to claim 1, wherein the purification catalystis a powder having a surface-to-weight ratio of 8 m² or more.
 7. Thepurification catalyst for exhaust gas according to claim 3, wherein thecarboxylic acid is malic acid.